Elution with 1,2-Hexanediol Enables Coupling of ICPMS with Reversed-Pase Liquid Chromatography under Standard Conditions

The inductively coupled plasma mass spectrometry (ICPMS) has been attracting increasing attention for many applications as an element-selective chromatographic detector. A major and fundamental limitation in coupling ICPMS with liquid chromatography is the limited compatibility with organic solvents, which has so far been addressed via a tedious approach, collectively referred to as the “organic ICPMS mode”, that can decrease detection sensitivity by up to 100-fold. Herein, we report 1,2-hexanediol as a new eluent in high-performance liquid chromatography–ICPMS which enables avoiding the current limitations. Unlike commonly used eluents, 1,2-hexanediol was remarkably compatible with ICPMS detection at high flow rates of 1.5 mL min–1 and concentrations of at least 30% v/v, respectively, under the standard conditions and instrumental setup normally used with 100% aqueous media. Sensitivity for all tested elements (P, S, Cl, Br, Se, and As) was enhanced with 10% v/v 1,2-hexanediol relative to that of 100% aqueous media by 1.5–7-fold depending on the element. Concentrations of 1,2-hexanediol at ≤30% v/v were superior in elution strength to concentrations at >90% v/v of the common organic phases, which greatly decreases the amount of carbon required to elute highly hydrophobic compounds such as lipids and steroids, enabling detection at ultra-trace levels. The proposed approach was applied to detect arsenic-containing fatty acids in spiked human urine, and detection limits of <0.01 μg As L–1 were achieved, which is >100-fold lower than those previously reported using the organic ICPMS mode. Nontargeted speciation analysis in Allium sativum revealed the presence of a large number of hydrophobic sulfur-containing metabolomic features at trace levels.


Figure S1
The elution of cholesterol sulfate under various compositions of organic solvents, namely, 1,2-hexanediol (A), isopropanol (B), and acetonitrile (C). Reasonable retention (i.e. k<20) was not achievable using any concentration of methanol or acetonitrile. Comparable elution strength to 30 % 1,2-hexanediol was achievable using 60 % v/v isopropanol. Due to the lack of a sufficiently strong chromophore in cholesterol sulfate and to confirm the observed elution patterns for this compound, detection was undertaken with a molecule-selective ESI-MS/MS detector in the negative mode using the mass transition 465 → 97. The column void time was 0.55 min.

Figure S2
The elution of benzene sulfonamide (LogP 0.3) and toluene sulfonamide (LogP 0.8) with low organic proportions from the C18 reversed-phase column. The chromatograms show the elution with 100 % aqueous mobile phase as well as 0.5-1.0 % v/v 1,2-hexanediol and 10-20 % v/v methanol. Detailed chromatographic conditions can be found in the Experimental section.

Figure S3
The relationship between retention factor and various concentrations of 1,2-hexanediol (3.0-15 % v/v) spanning the previously reported critical micelle concentration (cmc) of 0.7M (ca. 8.8 % v/v). Cloxacillin was chosen for this investigation as it maintained reasonable retention over a concentration range spanning cmc. Linear regression based on the entire dataset yielded r 2 = 0.9955. An increase in slope (ca. 10 %) corresponding to decreased retention by ca. 30 % is evident around the composition corresponding to cmc, which can be explained by the influence of the formation of micelles on the elution strength.

Figure S4
Comparing the backpressure under eluents containing 1,2-hexanediol with those containing methanol at comparable concentrations. The y-axis shows normalized backpressure values relative to 100 % aqueous eluent. Isopropanol showed similar patterns to 1,2-hexanediol (e.g.2.5-fold relative to pure water at 30 % v/v isopropanol). Note that this investigation was performed at 50 °C (for other chromatographic conditions see Experimental).

Figure S5
The appearance of the sampler and skimmer cones of the ICPMS/(MS) system following 3 hours of operation under various concentrations of 1,2-hexanediol (up to 25 % v/v) as an HPLC eluent at 0.25 mL min -1 flow rate. Note that the photos on the left were taken directly after cleaning the cones with a solution containing 1 % nitric acid under ultrasonication for 10 min. The discoloration that appears on the right-side photos also takes place without introducing an organic solvent. No black carbon build-up at the tips of the sampler and skimmer cones following exposure to 1,2-hexanediol was observed. The cones maintained the appearance shown for the entire study period.

Figure S6
Investigating the plasma stability and sensitivity with 1,2 hexanediol at 30 % v/v. The employed mobile phase solution contained1,2-hexanediol at 30 % v/v in water. Five mobile phase flow rates were tested (see graph). A make-up (argon) gas fixed at 0.25 L min -1 was used in all experiments. Nebulizer gas flow rate was lowered gradually starting from 0.80 L min -1 and the RF matching increased to yield a reflected power <3 W. Note that these experiments were performed with the AriMist ® nebulizer (max. operatable nebulizer gas flow rate 0.8 L min -1 ) and the 2.5 mm torch, as described in the experimental section. Similar experiments using different values for the make-up argon gas flow rate in the range of 0.30-0.40 L min -1 yielded similar patterns (i.e. peak sensitivity around 6.5-8.0 × 10 5 CPS at 0.9 L min -1 total carrier gas flow rate), except that some combinations of high mobile phase flow rates (≥0.75 mL min -1 ) and low nebulizer gas flow rates (<0.6 L min -1 ) and total carrier gas flow rate (<0.9 L min -1 ) resulted in reflected power spikes and plasma instability. These combinations were however associated with either decreased or no significant change (within ±20 %) in sensitivity.

Figure S7
Comparing the plasma carbon load based on the signal of the polyatomic species 40 Ar 12 C between methanol and comparable concentrations of 1,2-hexanediol at a mobile phase flow rate of 0.25 mL min -1 . The ICPMS/(MS) was operated in the no-gas mode. Note the significantly lower 40 Ar 12 C signal for 1,2-hexanediol relative to methanol despite the fact that the former has roughly double the carbon molarity (carbon molarity in pure methanol and 1,2-hexanediol is 25 and 48 M, respectively).

Figure S8
The detection of arsenic fatty acids 362 and 418 at 0.01 µg As L -1 in water. The mobile phase contained 10 % v/v 1,2-hexanediol and 0.1 % formic acid. The injection volume was 50 µL.